The primary objective of this study is to evaluate the efficacy of acyclovir (5%) cream vs. placebo cream in the abortion of herpes labialis lesions (i.e., prevention of development beyond the papular stage). The duration of the herpes labialis episode and time to healing for classical lesion (time from vesicle formation until loss of hard crust), for those subjects who develop vesicles, will be examined as a secondary variable. Other secondary measures will include pain, tenderness and viral shedding.